This invention relates to an apparatus and method for non-invasively detecting, digitally recording and processing cardiac generated electrical signals in the assessment of acute myocardial damage at the site of the victim's heart attack.
Electrical conduction in the human heart is initiated by a spontaneous electrical impulse at the sinoatrial node located at the top of the heart. This depolarising wave which spreads down the heart gives rise to heart muscle contraction and hence causes pumping of the blood. The ordered contraction of the heart muscle depends on an adequate supply of blood provided by the heart's own coronary arteries. Blockage "clot" of a coronary artery deprives a certain portion of cardiac muscle from receiving blood "ischaemia" and results in myocardial infarction. Myocardial ischaemia is a major cause of heart attacks and results in cardiac injury or even death to a high proportion of patients. Current medical intervention now provides "clot busting" thrombolytic drugs to remove the clot in the occluded coronary artery. These anti-clotting agents are given intraveneously on monitoring ischaemic damage. The potency or absorption rate of these drugs ensures rapid restoration of the ischaemic region by the reperfusion of blood. Since the majority of heart attacks occur outside hospital myocardial salvage will depend on the time taken to attend the patient and provide thrombolytic therapy.
A typical graph of cardiac generated potential of a normal heart beat is shown in FIG. 1, and consists of an isoelectric flat portion followed by a P wave, which is generated by the depolarisation of the atria, a QRS wave pattern, resulting from the depolarisation of the ventricles, and the T wave indicating repolarisation of the ventricles and termination of the heart beat. Initial ischaemic damage gives rise to the generation of "injury currents" through the chemical imbalance of these damaged muscle cells. This is reflected in the heart beat as elevation of the ST period as shown in FIG. 2 and the height provides an indication as to degree of damage. This electrical activity is at a maximum at the time of the heart attack and will change as infarction is caused or if thrombolytic therapy is provided.
Conventional electrocardiographs measure surface potentials from a limited number of locations i.e. a maximum of nine points. These twelve-lead electrocardiographic recorder/analyzers provide limited detection of ischaemic damage since they are primarily concerned with the interpretation of rhythm disorders. Heretofore it has only been considered necessary to monitor a small number of electrocardiograms in emergency situations in an attempt to detect myocardial ischaemia through some electrocardiograms showing sufficient elevated ST levels. This method, however, cannot detect all ischaemic regions throughout the heart and cannot provide assessment of the extent of the initial injury and therefore of subsequent recovery.
Clinical evaluations of myocardial infarction use blood enzyme tests and radio-isotope imaging which are invasive tests and cannot be used at the time of the attack and can only provide information hours after the event. They are, however, standardised clinical methods of reporting on myocardial infarction.
Body surface mapping systems are known. They are, however, concerned with providing detailed iso-potential contour plots and employ in the region of 200 leads. One known system, the Corazonix predictor BM-32 made by the Corazonix Corporation, of Oklahoma City, United States of America employs 32 leads. The system detects 32 electrocardiograms and, by extrapolation, and with further interpolation between these leads, provides a high definition of the geographical contour style mapping in the manner of a 192 lead system. This type of mapping system is then used in an attempt to differentiate normal patient distribution with suspected abnormal or infarct patients by using differencing maps. Articles by Robert L. Lux and others of the College of Medicine, University of Utah relate to the use of a large number of ECG leads. Their subsequent work shows that spatial redundancy may be achieved to reduce the 192 contour style lead system to 12 co-efficient waveforms. More recently the patent of Erwin R. John, 1990 (U.S. Pat. No. 4,974,598) employs multiple electrocardiogram statistical analysis in an attempt to provide a system or method for determining the presence of a wide range of heart disease conditions among a broader population. These current methods are concerned with the problems of contour style maps to identify abnormalities or sophisticated multiple statistical analysis in an attempt to identify a wide range of heart disorders. None of the known methods provides a technique in which the extent of the initial ischaemic injury, particularly within minutes of the commencement of a heart attack, can be determined.